1. Field of the Invention
The present invention relates to chemical vapor deposition apparatus with which thin films are deposited using chemical reactions of a reactive gas.
2. Related Art
A process whereby appropriate thin films are deposited on the surface of a substrate is required in the production of large scale integrated circuits (LSI) and liquid crystal displays (LCD). The use of a chemical vapor deposition (CVD) apparatus with which a thin film is deposited using chemical reactions of a reactive gas is common in thin film deposition processing, since it is possible to select the composition ratio of the thin film quite freely. A front outline view of a conventional chemical vapor deposition apparatus of this type is shown in FIG. 10. The CVD apparatus shown in FIG. 10 is furnished with a reactor 1 which is provided with a pumping system 11, a substrate holder 2 which supports the substrate 20 which is to be processed at a prescribed location within the reactor 1, a heater 3 with which said substrate 20 is heated to a set temperature and a source gas delivery system 4 which delivers a source gas to the surface of the heated substrate 20. A thin film is deposited on the surface of the substrate 20 using chemical reactions on or near said surface.
Much research has been carried out recently in connection with MOCVD (metal organic chemical vapor deposition) in which an organometallic compound which is liquid at normal temperature and pressure is used as the precursor for the deposition of metals. For example, in the field of metal materials for wiring purposes, copper (Cu) which has a low specific resistance with a high electro-migration resistance is regarded as being of significance as a wiring material of the next generation. Organometallic compounds which are liquid at normal temperature and pressure, such as copper (trimethylvinylsilyl)hexafluoroacetylacetonate (referred to hereinafter as Ch(hfac)(tmvs)), are used as the precursor.
A source gas delivery system for a liquid precursor of this type is illustrated in FIG. 10. The source gas delivery system 4 comprises the precursor reservoir 41 in which the liquid-state precursor is stored, the vaporizer 42 which vaporizes the liquid-state precursor, the gas delivery pipe 421 which supplies the vaporized source gas to the reactor 1 and the valve 422 which is fitted thereto, and a mass flow controller which is not shown in the drawing. The shower head 43 which is established inside the reactor 1 is connected to the end of the gas delivery pipe 421.
The shower head 43 is a disk-shaped part with an internal space which is connected to the gas delivery pipe 421. This shower head 43 has a large number of gas discharge holes 430 in the front surface which faces the substrate 20 which is being supported on the substrate holder 2. The source gas is blown out from the gas discharge holes 430 and enters the reactor 1.
A carrier gas delivery system 5, which introduces the carrier gas which is mixed with the source gas, is connected to the source gas delivery system 4 in order to introduce the source gas into the reactor 1 efficiently. The substrate holder 2 incorporates the heater 3 for heating the substrate 20 to the set temperature for bringing about chemical reactions on the surface of the substrate 20. The heater 12, which prevents liquefaction of the source gas, is established around the outer wall of the reactor 1.
The operation of the conventional chemical vapor deposition apparatus described above is explained by means of an example of the CVD deposition of copper using Cu(hfac)(tmvs) as the precursor. The substrate 20 is supported (clamped) onto the substrate holder 2 and the substrate 20 is heated to some 100 to 300.degree. C. by means of the heater 3. The Cu(hfac)(tmvs) is vaporized in the vaporizer 42.
The vaporized Cu(hfac)(tmvs) which has been mixed with hydrogen as a carrier gas is blown out from the gas discharge holes 430 of the shower head 43 and introduced into the reactor 1. The introduced Cu(hfac)(tmvs) gas reaches the substrate 20 and a film of Cu accumulates on the surface of the substrate 20 as a result of a series of chemical reactions including a degradation reaction which is caused by heat.
The source gas can be delivered uniformly to the whole surface of the substrate 20 by adjusting the arrangement of the gas discharge holes 430 of the shower head 43. The shower head 43 is one which is good in terms of the uniformity of the film thickness distribution and the uniformity of the film quality distribution (film quality characteristics distribution).
With conventional CVD apparatus of the type described above, increasing the deposition rate is of importance for increasing productivity. The deposition rate depends on the rate limiting conditions of the chemical reaction which results in the thin film. In general, at temperatures above a certain level, the rate of the chemical reaction is dominated by the delivery rate (mass-transfer limited rate determining process) and so the film growth rate is determined by the delivery rate of the precursor. Hence, with a chemical reaction, the rate of which is determined by the delivery rate, it is important that as much source gas as possible is delivered to the substrate surface as a precursor.
When the sufficient precursor is supplied at the substrate surface, the deposition rate is dominated by the surface reaction. In the case of Langmuir reaction type, when the partial pressure of the precursor at the substrate surface increase, the gap-filling property improve.
In those cases where a precursor which is a liquid at normal temperature and pressure is being used, the delivery rate of the source gas depends to a large extent on the vaporization efficiency of the vaporizer 42. The main factor which determines the vaporization efficiency is the pressure within the vaporizer 42. The vaporization efficiency is high when the pressure is low but it falls as the pressure increases.
With the conventional CVD deposition apparatus described above, the gas is blown out from the shower head 43 through the small discharge holes 430 and so the pressure inside the shower head 43 must inevitably rise. The interior of the vaporizer 42, which is on the upstream side of the shower head 43, is at an even higher pressure. Hence, it is difficult with conventional CVD apparatus in which a shower head 43 is used to increase the vaporization efficiency of the vaporizer 42. As a result, it is difficult to increase the deposition rate because the delivery rate of the precursor cannot be increased.
The pressure is raised where the gas flow flux is concentrated in the parts where the flow-way is small, such as in each of the gas discharge holes 430 of the shower head 43. As a result, the source gas may liquify. If the source gas liquifies then the reproducibility of the delivery rate of the source gas is poor and stable deposition becomes impossible. Furthermore, the gas discharge holes 430 become smaller due to the attachment of the liquified source gas and this results in a viscous circle and the pressure is increased even more. In the worst case, the gas discharge holes 430 may become blocked. Blockage of this type has the effect of rendering the delivery rate of source gas from each of the gas discharge holes 430 irregular.
Moreover, there is a further problem with the CVD apparatus shown in FIG. 10 in that a film, for example a Cu film, is liable to be deposited on the inside and outside of the shower head 43, and this is liable to become a source of dust contamination. The shower head 43 which has the gas discharge holes 430 has a large surface area on which a film can be deposited and, moreover, there are many corners at which film deposition may start.
The apparatus shown in FIG. 11 makes use of a cap-shaped gas delivery guide 44 instead of the shower head 43 shown in FIG. 10. The gas delivery guide 44 comprises a circular base plate part 441 and the side plate part 442 which is established around the edge of the base plate part 441 extending toward the substrate 20. The internal diameter of the side plate part 442 is a little larger than the diameter of the substrate 20. An opening 443 (referred to hereinafter as the gas delivery port) is established in the center of the base plate part 441 which is located on the same axis as the center of the substrate 20. The end of the gas delivery pipe 421 is connected to this gas delivery port 443. The construction is otherwise essentially the same as that shown in FIG. 10.
The gas delivery guide 44 shown in FIG. 11 introduces gas into the reactor 1 from a single gas delivery port 443 which is established in the center. The gas delivery port 443 is quite large when compared with a single gas delivery hole 430 of the shower head 43 shown in FIG. 10. Hence, the pressure in the pipe 421 from the vaporizer 42 to the reactor 1 can be somewhat lower than that in the apparatus shown in FIG. 10. Consequently, when compared with the apparatus shown in FIG. 10, a lower pressure can be maintained in the vaporizer, the vaporization efficiency can be held at a higher level, and the deposition rate is increased by the rise in the delivery efficiency of the source gas.
However, although the deposition rate was increased when deposition was carried out in practice using the apparatus shown in FIG. 11, the film thickness distribution was not uniform. A nonuniform film thickness distribution appeared with a tendency for the film thickness in the outer parts to be lower than the film thickness in the middle part of the substrate 20. This is a result of the nonuniformity of the thickness distribution of the boundary layer of fluid dynamics at the surface of the substrate 20. This is explained below with reference to FIGS. 12(a) and 12(b), hereinafter referred to as FIG. 12.
FIGS. 12(a) and 12(b) are diagrams which show the flow of source gas on the surface of the substrate in the apparatus shown in FIG. 11. When operating the apparatus shown in FIG. 11, the source gas 40 which has been mixed with the carrier gas flows in from the gas delivery port 443 toward the substrate 20. The source gas 40 flows on the surface of the substrate 20 so as to spread out from the center of the substrate 20 over a range of 360.degree. in the form of a laminar flow, and reaches the outer edge of the substrate 20.
Consider in detail the state of the gas flow at the surface of the substrate 20. The flow of source gas 40 along the surface of the substrate 20 is a macro-laminar flow due to the difference between the pressures in the vaporizer 42 and the gas delivery pipe 421 and in the reactor 1.
As shown in FIGS. 12(a) and 12(b), the flow rate at the surface of the substrate 20 is physically zero. Moreover, the flow rate is also almost zero in the region very close to the surface. In the region very close to the surface of the substrate 20 the macro-flow of the source gas 40 is almost nonexistent. The movement of source gas in this region is dominated by the "diffusion" of fluid dynamics due to thermal motion. The region 401 in the vicinity of the substrate where the source gas 40 moves mainly as diffusion is a region located between the substrate 20, which is a static solid (solid bulk), and the laminar gas flow region 402 where the source gas 40 is a fluid. This region is called the boundary layer 401 following the terminology of fluid dynamics.
Efficient delivery of the source gas 40 to the surface of the substrate 20 is important for depositing a film by chemical vapor deposition, as described above. The source gas 40 is delivered for the thermo-chemical reaction which occurs at the surface of the substrate 20 mainly by diffusion, as described above. The thickness of the boundary layer 401 has an effect on the delivery of the source gas 40 by diffusion. In those cases where the boundary layer 401 is thin, the surface of the substrate 20 is reached quickly by diffusion transfer across a thin cross section of the boundary layer 401. Hence, the delivery efficiency of the source gas 40 to the surface of the substrate is increased and the deposition rate is increased.
The source gas cannot reach the surface of the substrate 20 without travelling a long distance in those cases where the boundary layer 401 is thick. The delivery efficiency of the source gas 40 is reduced and the deposition rate is also reduced. In other words, if the thickness of the boundary layer 401 is uniform then the deposition rate on the surface of the substrate 20 is also uniform, and the film thickness distribution of the deposited thin film is also uniform.
The deposition rate and the film characteristics are important deposition characteristics, and control of the rate of the surface reaction (which is to say the conditions) is necessary for improving the latter.
An adequate supply is necessary to make the surface reaction rate limiting and, even if the flow rate is increased, if the boundary layer thickness is increased then diffusion control predominates and the deposition characteristics are not good. In order to eliminate this it is necessary to achieve an adequate supply rate (an adequate concentration in the vicinity of the boundary layer) and to keep the boundary layer below a certain thickness.
The fact that the film thickness is greater in the middle part of the substrate and thin in the outer parts of the substrate with film deposition using the apparatus shown in FIG. 11 is thought to be based on the phenomenon outlined above. In the middle part of the substrate, close to the gas delivery port 443, the effect due to the flow of source gas 40 is considerable and the boundary layer 401 is thin. In the outer edge part the effect of the flow of the source gas 40 is reduced since these parts are far removed from the gas delivery port and the boundary layer 401 is thicker. The nonuniform film thickness described above arises as a result of this situation.
This trend is the same in cases where the difference in pressure in the vaporizer 42 and the gas delivery pipe 421 and in the reactor 1 is large and the rate of flow of gas from the gas delivery port 443 is high. Even if the flow rate from the gas delivery port 443 is high, the boundary layer 401 in the center of the substrate simply becomes thinner, and the nonuniform thickness of the boundary layer 401 in the middle part of the substrate and outer part of the substrate cannot be eliminated.